1. Technical Field
The invention relates to optical waveguides in general and particularly to optical waveguides that employ materials having large nonlinear optical characteristics.
2. Related Technology
Integrated nonlinear devices have been sought for frequency conversion, particularly to generate optical radiation at wavelengths that are not readily generated by presently available laser devices.
Group III-V semiconductors that crystallize in a zinc blende lattice possess a large second-order nonlinear susceptibility, in excess of 100 pm/V. This quality, and their wide use in active optoelectronic devices have made III-V materials of interest for integrated nonlinear devices for frequency conversion.
Second-order nonlinear processes require phase matching between the three waves involved. One technique for phase matching involves the optical anisotropy of nonlinear crystals, a technique usually referred to as birefringent phase matching (BPM). However, bulk zinc blende materials are optically isotropic, which makes them not useful for BPM.
An alternative technique called quasi-phase matching (QPM) has also been widely investigated. QPM involves periodically inverting the sign of the nonlinear susceptibility. QPM is well-established in ferroelectric materials, such as lithium niobate. However, QPM in III-V semiconductors such as GaAs requires complex technologies and faces severe material problems, including waveguide loss. For additional discussion, see J. B. Khurgin, M. W. Pruessner, T. H. Stievater, and W. S. Rabinovich, “Suspended AlGaAs waveguides for tunable difference frequency generation in mid-infrared”, Optics Letters, Vol. 33, No. 24, pp. 2904-2906 (2008) and W. Denzer et al., “Mid-infrared generation and spectroscopy with a PPLN ridge waveguide”, Applied Physics B: Lasers and Optics, Vol. 86, No. 3, pp. 437-441, 2006.
Another approach to phase-matching is to take advantage of the artificial, or “form”, birefringence that can be attained in waveguides between orthogonally polarized modes. This approach may work in standard GaAs waveguides for as long as only relatively long wavelength photons (mid-IR) are involved in the process. However, the material dispersion becomes too large to be compensated by the form birefringence in applications using near-IR sources.
By the 1990s it had been found that that the relatively weak birefringence in typical AlxGa1-x As waveguides, in which the cladding and core differ slightly in composition, could be greatly enhanced if layers of Al2O3 with a small refractive index are introduced between the AlxGa1-x As layers via selective oxidation. Further discussion is found in A. Fiore et al., “Huge birefringence in selectively oxidized GaAs/AlAs optical waveguides”, Appl. Phys. Lett., Vol. 68, pp. 1320-22, (1996) and A. Fiore et al., “Phase-matched mid-infrared difference frequency generation in GaAs-based waveguides”, Appl. Phys. Lett., Vol. 71, pp. 3622-24, (1997).
A. Fiore et al., “Second-harmonic generation at λ=1.6 μm in AlGaAs/Al2O3 waveguides using birefringence phase matching”, Appl. Phys. Lett., Vol. 72, pp. 2942 (1998), describes second harmonic generation using birefringence phase matching. A. Fiore et al., “Phase matching using an isotropic nonlinear optical material”, Nature, Vol. 391, pp. 463-466, January 1998 describes difference frequency generation using this technique. A theoretical discussion is found in J. C. G. de Sande et al., “Phase-Matching Engineering in Birefringent AlGaAs Waveguides for Difference Frequency Generation”, Journal of Lightwave Technology, Vol. 20, Issue 4, pp. 651-660, (April 2002).
This technique has several challenges. First, O. Durand et al., “Contraction of aluminum oxide thin layers in optical heterostructures”, Appl. Phys. Lett., Vol. 83, pp. 2554 (2003) describes that wet oxidation of AlAs generally results in small grain poly-Al2O3 embedded in the AlAs matrix, which results in a composite AlAs/Al2O3 layer, and that the oxidation can result in a significant shrinkage of the layer thickness. In addition, A. Fiore et al., “Phase-matched mid-infrared difference frequency generation in GaAs-based waveguides”, Appl. Phys. Lett., Vol. 71, pp. 3622 (1997) describes that the tunability of the device is limited to about 50 cm−1 using temperature tuning.